The present invention relates to a method for controlling air-fuel ratio in an internal combustion engine and an apparatus for controlling the same and, more particularly, to a method and apparatus for controlling the fuel supply amount for a combustion chamber of an internal combustion engine. The invention is particularly concerned with control of the air-fuel ratio of an air-fuel mixture being supplied into the combustion chamber of the internal combustion engine during a transitional period in which an operational condition of the internal combustion engine changes.
The present invention relates to a method for controlling air-fuel ratio in an internal combustion engine and an apparatus for controlling the same, incorporating a plurality of sensors and an electronic control unit, or an electronic control computer which receives signals from various sensors, and which controls fuel injection and provides control of the internal combustion engine.
In a method for controlling air-fuel ratio in an internal combustion engine equipped with a fuel injection and control system, air and an appropriate amount of fuel is supplied by the fuel injection and control system during various and diverse operational conditions of the internal combustion engine so as to provide good engine operational characteristics, and an air-fuel ratio control apparatus operates in accordance with such an air-fuel ratio control method.
In an electric spark ignition type gasoline internal combustion engine for use in an automotive vehicle, a part of an injected fuel being supplied into the gasoline internal combustion engine adheres to an inner wall surface portion of an intake air flow passage and becomes an adhesion fuel film.
Such a fuel adhesion phenomenon will be explained referring to FIG. 7 which is a partial cross-sectional view showing a part of a gasoline internal combustion engine including an intake pipe, an intake valve, and a combustion chamber. Intake air flows into the combustion chamber from an intake pipe 8 passing through a value port in the vicinity of an intake valve 31. The gasoline fuel is injected into the above stated air flow from an injector 13.
A part of the injected fuel being supplied to the gasoline internal combustion engine 7 adheres to an inner wall portion of the intake air flow passage in the intake pipe 8 and forms an adhesion fuel film 32.
When the internal combustion engine an is operated at a stable operational condition internal for a long period, the fuel which adheres to an inner wall surface portion of an intake air flow passage in an intake pipe and the amount of the fuel evaporated from the intake air flow passage wall are equal or balanced; accordingly, a stable air-fuel ratio of the air-fuel mixture for the internal combustion engine can be maintained.
However, during a transient state when an operational condition (for example, engine speed, torque etc.) changes during operation of the internal combustion engine, then the air-fuel ratio of the mixture being supplied into the internal combustion engine is often incorrect.
To solve the above stated air-fuel ratio control problem, there has been developed a technique for controlling air-fuel ratio as described in U.S. Pat. No. 4,388,906. Namely, the amount of the fuel being supplied into the internal combustion engine is controlled or corrected in accordance with a calculation result obtained through the following model calculation formulas (1) and (2) which are described in the above stated U.S. patent. ##EQU1## wherein, G.sub.f : fuel supply amount
Q.sub.a : intake air flow amount PA1 (A/F).sub.o : target air-fuel ratio PA1 M.sub.f : fuel adhesion amount PA1 X: fuel adhesion rate PA1 .tau.: evaporation time constant. PA1 (i) An estimation execution value for an inner wall surface portion adhesion fuel amount is calculated distinctly and independently from the calculation processing for a basic fuel injection amount. PA1 (ii) A correction coefficient multiplied by the above stated basic fuel injection amount is calculated in accordance with the above stated estimation execution value for the inner wall surface portion adhesion fuel amount. PA1 (iii) The above stated correction coefficient is multiplied by the above stated basic fuel injection amount to obtain the final fuel injection amount. PA1 n: subscript indicating a calculation result of present time, PA1 n-1: subscript indicating a calculation result of a previous time.
In the above stated conventional air-fuel ratio control technique, the amount of fuel to be injected into the combustion chamber of the internal combustion engine is calculated in accordance with the above stated model calculation formulas (1) and (2) in which the amount M.sub.f the fuel which adheres to an inner wall surface portion of an intake air flow passage is estimated.
However, since it is necessary to perform the calculation processing in an electronic control unit which is selected taking into the consideration the required accuracy of the calculation of the fuel injection amount, a large burden is imposed on a central processing unit (CPU) in the electronic control unit. Further, there is a problem that it is necessary to provide a large memory capacity for programs and data in the electronic control unit.